For several new LED based lighting systems, light guides are used. Such a light guide, typically a flat or curved piece of transparent plastic or glass, usually serves two purposes. It guides light from a light source, e.g. one or more light emitting diodes (LEDs), to a desired spot, and helps it mixing the colors from individual red, green and blue LEDs. It is especially important with the mixing of colors, as non-perfect mixing results in colored edges and shadows, while white surfaces will not be white, but colored.
It is especially important with the mixing of colors, as non-perfect mixing results in colored edges and shadows, while white surfaces will not be white, but colored.
Light guides are used in many lighting applications, such as for general-purpose lighting or as a backlight for an LCD monitor or television. One such example is disclosed in International Patent Application WO 2004/008023 A1 in which a light guide device comprising an LED group as a light source, a light guide plate for color mixing and a light guide plate having a light output face is shown. The light guide device is provided as backlight source behind a liquid crystal display panel. It further shows a triangular prism for guiding light rays from the LED group to one end face of the light guide plate and triangular prisms for guiding light rays from the other end face of the light guide plate to one end face of the light guide plate.
Another possible application for these light guides is for instance in LED based ambient background lighting for television sets, such as a flat screen display panel. Light effects are generated around the TV that matches the image contents. The effect gives the impression of a larger virtual screen and a more immersive viewing experience. In addition, it reduces the strain on the eyes of the viewers.
In order to achieve this ambient background lighting effect it is known to have a lamp positioned behind a television set, which lamp emits light towards the wall. For these purposes it is common to use Cold Cathode Fluorescent (CCFL) lamps. For several reasons, a LED based version would be preferred. There are however known issues with color mixing and color uniformity associated with the use of LEDs. For instance, in order to be able to generate all colors, at least three different LEDs are required, i.e. one red, one green and one blue LED. The three LEDs are inherently positioned next to each other, and because of the different positions, the individual colors will not perfectly overlap each other in the output.
Using more LEDs per primary color is an alternative, yet this substantially increases the cost of a solution. Hence it would be preferably to have only a few, or even only one LED per color.